Evidence that dna can transform bacteria
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Evidence that DNA can transform bacteria. Frederick Griffith (1928) – Streptococcus pneumoniae bacteria – transformation Mouse Experiment Experiment proved that transformation can happen Avery and colleagues (1944) – announced transformation agent was DNA.

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Evidence that DNA can transform bacteria

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Evidence that dna can transform bacteria

Evidence that DNA can transform bacteria

  • Frederick Griffith (1928) – Streptococcus pneumoniae bacteria – transformation

  • Mouse Experiment

  • Experiment proved that transformation can happen

  • Avery and colleagues (1944) – announced transformation agent was DNA


Figure 16 2 can the genetic trait of pathogenicity be transferred between bacteria

Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they

have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule

and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:

CONCLUSION

EXPERIMENT

RESULTS

Living S

(control) cells

Living R

(control) cells

Heat-killed

(control) S cells

Mixture of heat-killed S cells

and living R cells

Mouse dies

Mouse healthy

Mouse healthy

Mouse dies

Living S cells

are found in

blood sample.

Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an

unknown, heritable substance from the dead S cells.

Figure 16.2 Can the genetic trait of pathogenicity be transferred between bacteria?


Evidence that viral dna can program cells

Evidence that viral DNA can program cells

  • Alfred Hershey and Martha Chase (1952) – bacteriophages or phages (viruses that infect bacteria) – discovered DNA is the genetic material NOT protein

  • Blender experiment


Figure 16 3 viruses infecting a bacterial cell

Phage

head

Tail

Tail fiber

DNA

100 nm

Bacterial

cell

Figure 16.3 Viruses infecting a bacterial cell


Additional evidence that dna is the genetic material

Additional evidence that DNA is the genetic material

  • Erwin Chargaff (1947) – Chargaff’s rules – The equivalences for any given species between the number of A and T and G and C bases areequal.

  • Analyzed DNA from different organisms

    • Humans 30.3% of bases were A’s

    • E. Coli 26% of bases were A’s


Evidence that dna can transform bacteria

  • Rosalind Franklin – (1950’s) – X-ray diffraction photo of DNA – helped Watson and Crick develop their model of DNA structure


Figure 16 6 rosalind franklin and her x ray diffraction photo of dna

(b)

(a) Rosalind Franklin

Franklin’s X-ray diffraction

Photograph of DNA

Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA


Structure of dna

Structure of DNA

  • Watson & Crick – (1953) – 1 page paper in the British journal Nature “Molecular Structure of Nucleic Acids: A Structure for Deoxynucleic acids”


Figure 16 1 watson and crick with their dna model

Figure 16.1 Watson and Crick with their DNA model


Figure 16 5 the structure of a dna strand

Sugar-phosphate

backbone

Nitrogenous

bases

5 end

CH3

O–

5

O

H

CH2

O

P

O

O

1

4

N

O–

N

H

H

H

H

H

O

2

3

H

Thymine (T)

O

H

H

CH2

O

O

P

N

O

N

H

O–

H

N

H

H

H

N

N

H

H

Adenine (A)

H

H

O

H

N

CH2

O

O

P

H

O

O–

N

H

N

H

H

H

O

H

Cytosine (C)

O

5

H

CH2

O

N

P

O

O

O

1

4

O–

H

N

H

Phosphate

H

H

N

2

H

3

DNA nucleotide

N

H

OH

N

Sugar (deoxyribose)

3 end

H

H

Guanine (G)

Figure 16.5 The structure of a DNA strand


Figure 16 7 the double helix

5 end

G

C

O

OH

A

T

Hydrogen bond

P

3 end

–O

O

T

A

OH

O

H2C

A

T

1 nm

O

O

CH2

O

C

G

P

O

O–

–O

3.4 nm

O

P

C

G

O

O

H2C

O

G

C

T

A

O

CH2

O

O

G

C

P

O

O–

–O

O

P

O

O

H2C

O

G

C

T

A

O

CH2

O

O

P

T

A

O

–O

O–

O

P

O

A

T

O

H2C

O

A

T

T

A

O

CH2

OH

O

O–

3 end

P

O

G

C

O

0.34 nm

5 end

T

A

(a) Key features of DNA structure

(b) Partial chemical structure

(c) Space-filling model

Figure 16.7 The double helix


Figure 16 8 base pairing in dna

N

O

H

CH3

N

N

N

N

H

Sugar

N

N

O

Sugar

Adenine (A)

Thymine (T)

H

O

N

H

N

N

N

H

N

Sugar

N

N

N

O

H

Sugar

H

Cytosine (C)

Guanine (G)

Figure 16.8 Base pairing in DNA

H


Unnumbered figure p 298

Purine + Purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width

Consistent with X-ray data

Unnumbered Figure p. 298


Dna replication section 16 2

DNA Replication Section 16.2

  • Semi-conservative model – each of the two daughter molecules will have one old strand, derived from the parent molecules, and one newly made strand


Figure 16 9 a model for dna replication the basic concept layer 1

A

T

C

G

T

A

A

T

G

C

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

Figure 16.9 A model for DNA replication: the basic concept (layer 1)


Figure 16 9 a model for dna replication the basic concept layer 2

A

A

T

T

C

G

C

G

T

A

T

A

A

A

T

T

G

G

C

C

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(b) The first step in replication is separation of the two DNA strands.

Figure 16.9 A model for DNA replication: the basic concept (layer 2)


Figure 16 9 a model for dna replication the basic concept layer 3

T

A

A

A

T

T

T

A

G

C

C

G

C

C

G

G

A

T

A

T

T

A

A

T

T

A

A

A

T

T

T

A

C

G

G

G

C

C

C

G

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

(b) The first step in replication is separation of the two DNA strands.

Figure 16.9 A model for DNA replication: the basic concept (layer 3)


Figure 16 9 a model for dna replication the basic concept layer 4

T

A

A

A

A

T

A

T

T

T

T

A

G

C

G

C

C

C

C

G

G

G

G

C

A

A

T

T

T

T

A

A

A

A

T

T

T

A

A

A

A

T

A

T

T

T

T

A

C

G

G

G

G

C

G

C

C

C

C

G

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

(d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

(b) The first step in replication is separation of the two DNA strands.

Figure 16.9 A model for DNA replication: the basic concept (layer 4)


Figure 16 10 three alternative models of dna replication

First

replication

Second

replication

Parent cell

(a)

Conservative

model. The two

parental strands

reassociate

after acting as

templates for

new strands,

thus restoring

the parental

double helix.

(b)

Semiconserva-

tive model.

The two strands

of the parental

molecule

separate,

and each functions

as a template

for synthesis of

a new, comple-

mentary strand.

(c)

Dispersive

model. Each

strand of both

daughter mol-

ecules contains

a mixture of

old and newly

synthesized

DNA.

Figure 16.10 Three alternative models of DNA replication


Figure 16 13 incorporation of a nucleotide into a dna strand

New strand

Template strand

3’ end

5’ end

3’ end

5’ end

Sugar

A

T

A

T

Base

Phosphate

C

G

C

G

G

G

C

C

A

T

A

T

OH

P

P

P

P

3’ end

P

Pyrophosphate

C

C

OH

2 P

Nucleoside

triphosphate

5’ end

5’ end

Figure 16.13 Incorporation of a nucleotide into a DNA strand


Figure 16 12 origins of replication in eukaryotes

Origin of replication

Parental (template) strand

0.25 µm

Daughter (new) strand

1

Replication begins at specific sites

where the two parental strands

separate and form replication

bubbles.

Bubble

Replication fork

2

The bubbles expand laterally, as

DNA replication proceeds in both

directions.

3

Eventually, the replication

bubbles fuse, and synthesis of

the daughter strands is

complete.

Two daughter DNA molecules

(a)

In eukaryotes, DNA replication begins at many sites along the giant

DNA molecule of each chromosome.

(b)

In this micrograph, three replication

bubbles are visible along the DNA of

a cultured Chinese hamster cell (TEM).

Figure 16.12 Origins of replication in eukaryotes


Figure 16 14 synthesis of leading and lagging strands during dna replication

DNA pol Ill elongates

DNA strands only in the

5 3 direction.

One new strand, the leading strand,

can elongate continuously 5 3

as the replication fork progresses.

2

3

1

4

3

5

Parental DNA

5

The other new strand, the

lagging strand must grow in an overall

3 5 direction by addition of short

segments, Okazaki fragments, that grow

5 3 (numbered here in the order

they were made).

3

Okazaki

fragments

2

3

1

5

DNA pol III

Template

strand

DNA ligase joins Okazaki

fragments by forming a bond between

their free ends. This results in a

continuous strand.

Leading strand

Lagging strand

3

1

2

Template

strand

DNA ligase

Overall direction of replication

Figure 16.14 Synthesis of leading and lagging strands during DNA replication


Figure 16 15 synthesis of the lagging strand

6

7

1

5

2

4

3

3

5

3

5

Templatestrand

Primase joins RNA nucleotides into a primer.

DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.

RNA primer

3

5

3

1

5

After reaching the next RNA primer (not shown), DNA pol III falls off.

Okazakifragment

3

3

5

1

5

After the second fragment is primed. DNA pol III adds DNAnucleotides until it reaches the first primer and falls off.

5

3

3

5

2

1

DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2.

5

3

3

5

2

1

DNA ligase forms a bond between the newest DNAand the adjacent DNA of fragment 1.

The lagging strand in this region is nowcomplete.

5

3

3

5

2

1

Overall direction of replication

Figure 16.15 Synthesis of the lagging strand


Table 16 1 bacterial dna replication proteins and their functions

Table 16.1 Bacterial DNA replication proteins and their functions


Figure 16 16 a summary of bacterial dna replication

Overall direction of replication

Lagging

strand

Leading

strand

Helicase unwinds the

parental double helix.

Origin of replication

1

Molecules of single-

strand binding protein

stabilize the unwound

template strands.

The leading strand is

synthesized continuously in the

5 3 direction by DNA pol III.

2

3

Leading

strand

Lagging

strand

OVERVIEW

DNA pol III

Leading

strand

5

Replication fork

DNA ligase

DNA pol I

3

Primase

2

Parental DNA

Lagging

strand

DNA pol III

1

Primer

3

Primase begins synthesis

of RNA primer for fifth

Okazaki fragment.

4

3

5

4

DNA pol I removes the primer from the 5 end

of the second fragment, replacing it with DNA

nucleotides that it adds one by one to the 3’ end

of the third fragment. The replacement of the

last RNA nucleotide with DNA leaves the sugar-

phosphate backbone with a free 3 end.

DNA pol III is completing synthesis of

the fourth fragment, when it reaches the

RNA primer on the third fragment, it will

dissociate, move to the replication fork,

and add DNA nucleotides to the 3 endof the fifth fragment primer.

DNA ligase bonds

the 3 end of the

second fragment to

the 5 end of the first

fragment.

5

6

7

Figure 16.16 A summary of bacterial DNA replication


Proofreading and repairing dna

Proofreading and repairing DNA

  • Errors do occur

    • 1 in 10 billion nucleotides on entire DNA

    • 1 in 100,000 for incoming nucleotides

  • Proofreading is done by DNA pol III as it attaches new nucleotides

  • Mismatch repair cells use special enzymes to fix mismatched nucleotides

    • A-C for example


Figure 16 17 nucleotide excision repair of dna damage

A thymine dimer

distorts the DNA molecule.

1

3

4

2

A nuclease enzyme cuts

the damaged DNA strand

at two points and the

damaged section is

removed.

Nuclease

Repair synthesis by

a DNA polymerase

fills in the missing

nucleotides.

DNA

polymerase

DNA

ligase

DNA ligase seals the

Free end of the new DNA

To the old DNA, making the

strand complete.

Figure 16.17 Nucleotide excision repair of DNA damage


Telomeres

Telomeres

  • Repeated units of bases

    • TTAGGG in humans

  • Do NOT contain genes

  • They protect the genes from being eroded (getting shorter and shorter) through DNA replication rounds


Figure 16 18 shortening of the ends of linear dna molecules

5

End of parental

DNA strands

Leading strand

Lagging strand

3

Last fragment

Previous fragment

RNA primer

5

Lagging strand

3

Primer removed but

cannot be replaced

with DNA because

no 3 end available

for DNA polymerase

Removal of primers and

replacement with DNA

where a 3 end is available

5

3

Second round

of replication

5

3

New leading strand

New lagging strand 5

3

Further rounds

of replication

Shorter and shorter

daughter molecules

Figure 16.18 Shortening of the ends of linear DNA molecules


Figure 16 19 telomeres

1 µm

Figure 16.19 Telomeres


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